Method and apparatus for suppressing drillstring vibrations

ABSTRACT

Apparatus and methods are described for isolating in a drillstring a device from acoustic noise by introducing between the device and a source of acoustic vibration an noise isolating apparatus, wherein the noise isolating apparatus has adjacent zones of different acoustic impedance. The zones are preferably created by zones of different thickness and designed such that noise suppression occurs in a predetermined range of acoustic frequencies. In a preferred application the apparatus and method is used in combination with drillstring telemetry.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to apparatus and methods forreducing noise generated by elastic waves travelling through a tubularplaced in a wellbore. More specifically, it relates to such noisereducing apparatus and methods for transmitting signals from asubterranean location to the surface using modulated elastic waves,preferably torsional waves.

[0003] 2. Description of Related Art

[0004] Background of the invention is the drilling of subterraneanwells. Wells of the type commonly used for hydrocarbon or geothermalexploration are typically less than 30 cm (12 inches) in diameter and onthe order of 2 to 8 km (1.5 to 5 miles) long. These wells are drilledusing drillstrings assembled from relatively light sections (either 30or 45 feet long) of drill pipe that are connected end-to-end by tooljoints, additional sections being added to the uphole end as the holedeepens.

[0005] In addition to this classical method of drilling wells, attemptsare made using a continuous reeled drillstring (“coiled tubing) carryinga downhole motor and a drill bit to perform drilling operation. Itremains to be seen whether or not these attempts result in a change ofdrilling technology in the future.

[0006] The downhole end of the drillstring typically includes a drillcollar, a weight assembled from sections of collar pipes with increasingdiameter having an overall length on the order of 300 meters (1000feet). A drill bit is attached to the downhole end of the drill collar,the weight of the collar causing the bit to bite into the earth as thedrillstring is rotated from the surface. Sometimes, downhole mud motorsor turbines are used to turn the bit.

[0007] Drilling fluid is pumped from the surface to the drill bitthrough an axial hole in the drillstring. This fluid removes thecuttings from the hole, provides a hydrostatic head which controls theformation gases, provides a deposit on the wall to seal the formation,and sometimes provides cooling for the bit.

[0008] Communication between downhole sensors and the surface has longbeen desirable. This communication is for example an integral part ofmethods known as Measurement-While-Drilling (MWD) andLogging-While-Drilling (LWD). Various methods that have been tried forthis communication include electromagnetic radiation through the groundformation, electrical transmission through an insulated conductor,pressure pulse propagation through the drilling mud, and acoustic wavepropagation through the metal drillstring. Each of these methods hasdisadvantages associated with signal attenuation, ambient noise, hightemperatures and compatibility with standard drilling procedures.

[0009] The most commercially successful of these methods has been thetransmission of information by pressure pulse in the drilling mud.However, attenuation mechanisms in the mud limit the effectivetransmission rate to less than 10 bits per second, even though higherrates have been achieved in laboratory tests.

[0010] This invention is directed towards the acoustical transmission ofdata through the metal drillstring. The history of such efforts isrecorded in columns 2-4 of U.S. Pat. No.4,293,936, issued Oct. 6, 1981,of Cox and Chancy. As reported therein, the first efforts were in thelate 1940's by Sun Oil Company, which organization concluded there wastoo much attenuation in the drillstring for the technology at that time.U.S. Pat. No. 3,252,225, issued May 24, 1966, of E. Hixon concluded thatthe length of the drill pipes and joints had an effect on thetransmission of energy up the drillstring. Hixon determined that thewavelength of the transmitted data should be greater than twice andpreferably four times the length of a section of pipe.

[0011] In 1968 Sun Oil tried again, using repeaters spaced along thedrillstring and transmitting the best frequency range, one withattenuation of only 10 dB/1000 feet. A paper by Thomas Barnes et al.,“Passbands for Acoustic Transmission in an Idealized Drillstring”,Journal of Acoustical Society of America, Vol.51, No.5, 1972, 15 pages1606-1608, was consulted for an explanation of the field-test results,which were not totally consistent with the theory. Eventually, Sun wentback to random searching for the best frequencies for transmission, anunsuccessful procedure.

[0012] The aforementioned Cox and Chancy patent concluded from theirinterpretation of the measured data obtained from a field test in apetroleum well that the Barnes model must be in error, because thecentre of the passbands measured by Cox and Chancy did not agree withthe predicted passbands of Barnes et al. The patent uses acousticrepeaters along the drillstring to ensure transmission of a particularfrequency for a particular length of drill pipe to the surface.

[0013] More recent patents related to the background of the presentinvention include U.S. Pat. No. 4,314,365, issued Feb. 2, 1982, Thispatent discloses a system similar to Hixon for transmitting acousticfrequencies between 290 Hz and 400 Hz down a drillstring.

[0014] U.S. Pat. No. 4,390,975, issued Jun. 28, 1983, of E. Shawhan,notes that ringing in the drillstring could cause a binary “zero” to bemistaken as a “one”. This patent proposes using a delay to allow thetransients to ring down before transmitting subsequent data.

[0015] U.S. Pat. No. 4,562,559, issued Dec. 31. 1985, of H. E. Sharp etal, uncovers the existence of “line structure” within the passbands;e.g., “such fine structure is in the nature of a comb with transmissionvoids or gaps occurring between teeth representing transmission bands,both within the overall passbands.” Sharp attributes this structure to“differences in pipe length, conditions of tool joints, and the like.”The patent proposes a complicated phase shifted wave with a broaderfrequency spectrum to bridge these gaps.

[0016] U.S. Pat. No. 5,128,901, issued Jul. 7, 1992, of D. S. Drumhellerprovides further apparatus and methods for transmitting data along adrillstring by use of a modulated continuous acoustical carrier wavecentred within a passband of the drillstring. Noise is reduced bymultiplying each frequency component of the signal by a factor whichdepended on the length of the drill pipe section. To eliminate the finestructure of the passbands, echoes are suppressed at each end of thedrillstring employing an arrangement of magneto-strictive ring elementsas transducers. The method is described to fail for shorter drillstringswith a length of below 3500 feet (1200 m).

[0017] S.U. Patent Application No. 1687759 A1, apparently describesmeans for improving the drilling efficiency. A hollow cylinder ofcomposite material is attached to the outer surface of the drillstring.The cylinder is a stack of rings, which absorb elastic wave energy,cause multiple reflections, and disrupt the wave front of an elasticwave. It should be noted that the patent does not relate to the field ofdownhole data transmission.

[0018] The United Kingdom Patent application GB 2311427 provides amechanical filter to reduce transmission of longitudinal compressionalwaves along a drill string. The filter a section of tube havingrelatively compliant walls to which are attached one or more masses viasprings. The masses act on the spring to provide high impedance pointsat particular frequencies.

[0019] In view of the above cited prior art it is an object of theinvention to provide improved noise reduction and cancellation apparatusand methods for downhole telemetry systems. The system should becompatible with or independent from the various transmission andencoding methods. It is a particular object of the invention to providesuch a system for torsional wave telemetry using the drillstring asmedium.

SUMMARY OF THE INVENTION

[0020] The objects of the invention are achieved by methods andapparatus as set forth in the appended independent claims.

[0021] Known analysis of the behaviour of stress waves travellingthrough a drillstring reveals that, when this stress wave encounters achange of impedance (e.g. a change of cross sectional area, or a changeof material properties), some of the wave energy will be transmittedthrough the impedance contrast, and some will be reflected. The amountof reflected energy depends upon the size of the impedance contrast. Theeffect of impedance changes on the transmission of stress waves alongthe drillstring can be quite large. For example, the periodic impedancechanges caused by the presence of tool joints in a drillstring leads topassbands in the transmission response, as described in theaforementioned patents.

[0022] A basic feature of the present invention is to introduce in thedrillstring impedance changes of particular sizes and spacing so as todeliberately create stopbands over particular frequency ranges in orderto suppress noise in these frequency ranges.

[0023] A preferred location for introducing the impedance changes issituated between the drill bit and any device which has to be protectedfrom noise in particular frequency ranges. The device could be asensitive mechanical, hydraulical, or electrical unit. In a preferredembodiment of the invention, the device is a transducer section whichgenerates telemetry signals or a downhole drillstring sensor. With thetransducer section operating in at least one of the created stopbands,noise arising from the drilling process is effectively cancelled fromthe transmitted signals.

[0024] The impedance changes are preferably introduced by means of abaffle sub, i.e., a specifically designed part of the drillstring. It ispreferred to have introduce moving parts into the drillstring.Therefore, in a preferred embodiment of the invention the baffle sub hasno parts which move relatively to adjacents parts of the sub. Hence nodrilling fluid or debris can enter between moving parts of the baffleand cause friction or other undesired effects.

[0025] The width, depth and position of the stop band created by thebaffle sub depend upon its geometry, and the drillstring it is connectedto. In general, to move the stop band to a lower frequency requires thesub to be longer. The depth of the stop band depends upon both thelength of the sub and the size of the impedance contrasts. For torsionalwaves the impedance contrasts depend upon the ratios of the polarmoments of inertia of the different sections of the sub, which areproportional to the radii raised to the power 4. For axial waves theimpedance contrasts depend upon the ratios of the cross sectional areasof the sub sections, which are proportional to the radii squared. Hencethe stop bands that can be achieved for torsional waves are much deeperthan those that can be achieved for axial waves.

[0026] Since the sub must be rugged enough to withstand the drillingprocess, there will be constraints on the minimum outside diameter. Theconstraints on the maximum outside diameter will be dictated by holesize and well geometry.

[0027] In a preferred embodiment of the invention, the impedance changesare generated by introducing a metal sub of essentially cylindricalshape into the drillstring. Whereas preferably the inner diameter of thesub is constant, its outer diameter varies along the longitudinal axisof the cylinder. By filling void zones in the outer surface of the subwith a suitable material, the outer diameter can be rendered uniformagain while preserving the impedance changes.

[0028] The presence of other acoustic impedance contrasts in thevicinity of the baffle sub, for example because of other BHA components,will have an effect on the response of the system. It may be desirableto consider the drillstring geometry when designing the sub. In this waythe performance can be optimized for a particular bottom hole assembly(BHA). Therefore, it may be advantageous to construct the sub from anumber of smaller components of the appropriate lengths and diameters.Thus the geometry of the sub can be easily changed when the BHA ischanged.

[0029] Without dissipating elements or material in the baffle sub, theenergy which is unable to pass the baffle sub is reflected back towardsthe bit.

[0030] These and other features of the invention, preferred embodimentsand variants thereof, and further advantages of the invention willbecome appreciated and understood be those skilled in the art from thedetailed description and drawings following below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 shows an example of a drillstring;

[0032]FIG. 2 shows the transmission response of a drillstring fortorsional waves;

[0033]FIG. 3A shows a schematic cross-section through a drillstringincluding a baffle sub in accordance with an example of the presentinvention;

[0034]FIG. 3B shows an enlarged view of the baffle sub of FIG. 3A;

[0035]FIG. 4 shows the transmission response of a drillstring fortorsional waves without and with (dashed line) the baffle sub of FIG.3A; and

[0036]FIG. 5 shows an alternative arrangement for the baffle sub withina bottom hole assembly (BHA).

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0037] The invention is based on based on the known analysis of waveenergy travelling through the drillstring as described for example by D.S. Drumheller in: “The Propagation of Sound Waves in drillstrings”, J.Acoust. Soc. Am., Vol. 97, pp. 2116-2125. Wave energy can travel alongthe drillstring as axial or torsional stress waves. As the analysisshows, the periodic structure of the drillstring gives rise to a bandstructure leaving distinct frequency bands for energy transmission(passbands) while suppressing energy in other frequency ranges(stopbands). The analysis further shows that the passbands have a finestructure which however is of no concern for the present application.

[0038] Any signal transmitting or telemetry process using stress wavesalong the drillstring as a medium is hampered by the noise which isgenerated in the drillstring by the drilling process itself. Severalattempts to overcome this problem are described in the references citedabove. However, none of the known solutions provided the necessaryrobustness and versatility required under the hostile ambient conditionsof a wellbore during a drilling operation. The following examplesdemonstrate how a the drilling noise can be cancelled from thetransmission path used for the telemetry signals.

[0039] Basis for the following analysis is a typical drillstring asshown in FIG. 1. The drillstring comprises several sections including a780 m long section of 5 inch 19.5 lb/ft drill pipe 101, followed by 112m of 5 inch 50 lb/ft heavy wall drill pipe 102, 100 m of 6 inch drillcollar 103 and finally a 100 m long section of 8 inch drill collar 104terminated by the drill bit 105.

[0040] The internal structure of the drillstring gives rise to atransmission response as depicted in FIG. 2. Though the analysis is madefor torsional waves, a similar response spectrum can be derived forother wave forms (e.g. axial waves). According to the transmissionresponse of the drillstring signal transmission should be possible inthe frequency ranges 0 to 120 Hz (first passband), 180 to 260 Hz (secondpassband) and above 370 Hz (third passband). Further passband are notconsidered in the present example but could equally be employed forsignal transmission.

[0041] Referring now to FIG. 3A, a baffle sub 303 in accordance with theinvention is shown. In the example the baffle sub 303 is part of thebottom hole assembly (BHA) 30, located near the end of the 8 inch drillcollar section between the signal transmission unit 302 and severalmeasurement-while-drilling (MWD) units 304. The BHA further comprisesdrill collars 305 and the drill bit 306. The location of the baffle subis chosen such that during drilling operation the transmission unit 302is, with respect to the passband used for signal transmission,acoustically isolated from noise stemming from drill bit. Morespecifically, the baffle sub generates a stopband which at least partlyoverlaps with the passband used for signal transmission.

[0042] The baffle design, as the design for all other apparatus inaccordance with the present invention, is apparently dominated by fourparameters: the size of the impedance contrasts, the ratio of lengths ofthe sections with high and low impedance (e.g. thick and thin sections),the absolute length of either of these sections, and the total length ofthe apparatus or baffle sub.

[0043] The depth of the stopband thus generated is controlled by thesize of the impedance contrast between the thick and thin sections. Asthe depth of the stopband increases with the impedance contrast, it canbe seen as a first design rule to make this contrast as large aspossible. When using a modified drill collar as baffle sub, the freedomto increase the impedance contrast is limited by maximal outer diameterof the drill collar, which is controlled by the well dimension, and itsminimal inner diameter, which is limited by the required strength. Theabsolute values of these limits vary in dependence of well parametersand the material or materials (in case of a composite material)of thesub.

[0044] The depth of the stopband also increases as the overall length ofthe sub increases, leading to a second design rule.

[0045] However, the width and position of the stopband(s) is bestcontrolled by using either the absolute width(s) of the section(s) withhigh or low impedance, respectively, or a ratio of the widths of thesection(s) or a combination of both methods.

[0046] The positions of the stopbands can be calculated for a periodicstructure using for example the method taught by T. G. Barnes and B. R.Kirkwood in: Passbands for Acoustic Transmission in an Idealized DrillString, J.Acoustic.Soc.Am. Vol. 51(2), pp.1606-1608. Following thismethod, the following expression for the phase of a torsional wave canbe obtained:

[1] cos W=cos kl₁ cos kl₂−M sin kl₁ sin kl₂,

[0047] with $\begin{matrix}{{M = {\frac{1}{2}\left\lbrack {\frac{r_{3}^{4} - r_{0}^{4}}{r_{2}^{4} - r_{1}^{4}} + \frac{r_{2}^{4} - r_{1}^{4}}{r_{3}^{4} - r_{0}^{4}}} \right\rbrack}}{and}} & \lbrack 2\rbrack \\{{k = \frac{\omega}{c}},} & \lbrack 3\rbrack\end{matrix}$

[0048] where c is the wave speed and ω is the frequency (in radians persecond). The lengths l₁ and l₂ denote the lengths of the two sectionwith different impedance, and the radii r_(i) denote inner and outerradius of the two sections, respectively (for details see FIG. 3B).

[0049] Stopbands occur when |cos W|>1. In general, the depth of thestopbands increases with the number of repeated units in the periodicstructure, and with increasing impedance contrast between the twosections. If l_(t) is the total length of the repeated unit, i.e.,l_(t)=l₁+l₂, then for any given number of r₀ to r₃ the maximumattenuation in the stopbands occurs when l₁=l₂.

[0050] Referring to FIG. 3B, details of the baffle sub 303 are shown inFIG. 3B. The sub is produced from two eight inch (i.e. r₃=20 cm) drillcollars. The total length of the sub is 20 m. Four circumferentialgrooves 307 are cut into the outer surface of the sub. The grooves havea uniform depth of approximately two inches (5 mm). The width, i.e. l₁,of the grooves and their mutual distance (l₂) is four meters. The innerradii (i.e. r₀ and r₁) in the given example are equal (4 cm), althoughin general they might differ as indicated in FIG. 3B. In accordance withthe description above, the location of the stopband is controlled by thewidth of the grooves and the distance between two adjacent grooves. Forsome applications, it may be advantageous to fill the grooves with anepoxy resin.

[0051] After introduction of the baffle sub, the shape of thetransmission response alters. This effect is illustrated by FIG. 4,which compares the original response function 40 with the new responsefunction 41. Notably, the second passband is reduced on average byapproximately 30 dB.

[0052] Even though the above example refers to torsional wavesuppression, it is immediately obvious that the apparatus according tothe present invention with little adaptation applies to compressionalwave suppression and can be advantageously used in other fields whichrequire an acoustic isolation between two or more sections of thedrillstring, e.g. look-ahead seismic measurements.

[0053] In FIG. 5, there is shown an arrangement 50 more suitable forlook-ahead seismic measurements. In this arrangement, the baffle sub 503is located between the drill bit 506 and the MWD section 504 so thatacoustic energy reflected from the formation and detected usingreceivers within the MWD section is not contaminated by acoustic noisetravelling along the drillstring

I claim:
 1. A method for isolating, in a drillstring, a device fromacoustic noise, said method comprising the step of introducing betweensaid device and a source of acoustic vibration a noise isolating sectionwherein said noise isolating section essentially consists of adjacentzones of different acoustic impedance such that noise propagation issuppressed in at least one predetermined frequency band.
 2. The methodof claim 1, wherein the device is a downhole sensor.
 3. The method ofclaim 1, wherein the device is a telemetry unit fortransmitting/receiving signal from and/or to another location.
 4. Themethod of claim 1, wherein the device is a telemetry unit using inoperation torsional waves for transmitting/receiving signal from and/orto another location.
 5. The method of claim 1, wherein the isolatingsection comprises a cylindrical base with adjacent zones of varying wallthickness.
 6. Apparatus for isolating in a drillstring a device fromacoustic noise, said apparatus essentially consisting of adjacent zonesof different acoustic impedance such that noise propagation issuppressed in at least one predetermined frequency band.
 7. Theapparatus of claim 6, wherein adjacent zones of different impedance aregenerated by zones of different thickness.
 8. The apparatus of claim 6,wherein adjacent zones of different impedance are generated by zonescomprising different materials.
 9. The apparatus of claim 6, wherein thezones are arranged periodically.
 10. The apparatus of claim 6,essentially consisting of a cylindrical body of steel with a periodicpattern of grooves.
 11. The apparatus of claim 10, wherein the groovesare filled with a second material so as to maintain a uniform inner andouter diameter of the cylindrical body of steel.